Wattage won't directly relate to your speed, since things like aerodynamic efficiency, terrain, weather, and weight will all play a role in how fast you go. For example, my roommate (a triathlete) and one of the time trialists on our cycling team go about the same time for a 40k (~57 min....a lot faster than me! :D). My roommate (145 lbs) puts out about 310W at LT, while the cyclist (210 lbs) puts out nearly 400W at the same speed. You can get a rough estimate of where you stand with these calculators:

http://www.kreuzotter.de/english/espeed.htm

http://www.analyticcycling.com/ForcesPower_Page.html

I think it depends what gear you are in? You can push a light gear really fast and go fast, but have a low wattage, or you can be pushing a heavy gear and going slower but have a higher wattage. You should really be concered with the wattage as it pertains to what sort of effort you are looking for, not what speed it correlates to. i.e. if your LT is 300 watts and you want to do a good endurance ride you should be looking at 200-220 in watts. I never look at speed, just watts. A power meter is most useful to measure your efforts and make sure you don't go out too hard and blow up, or if there is a particular zone you want to be in, use it to get there and stay there.